Fiber optic sensor and methods and apparatus relating thereto

ABSTRACT

There is disclosed fiber optic sensor which detects a sample in contact with the sensor by surface plasmon resonance (SPR) measurements, as well as methods and apparatus relating thereto. The fiber optic SPR sensor of this invention employs a limited range of incident angles and uses incident light having multiple wavelengths. In preferred embodiments, both an in-line transmission-based fiber optic SPR sensor and a terminated reflection-based fiber optic SPR sensor are disclosed. The fiber optic SPR sensor includes a surface plasmon supporting metal layer in contact with an exposed portion of the optical fiber core, and may optionally contain one or more additional layers deposited on the surface plasmon supporting metal layer. In further embodiments, methods are disclosed for detecting a sample by contacting the sample with the fiber optic SPR sensor of this invention, as well as sensing apparatus which contain the fiber optic SPR sensor in combination with a source of electromagnetic radiation of multiple wavelengths and a detection device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.08/584,960, filed Jan. 11, 1996, now U.S. Pat. No. 5,647,030, which is acontinuation of U.S. patent application Ser. No. 08/307,185, filed Sep.16, 1994, abandoned; which is a continuation of U.S. patent applicationSer. No. 08/003,224, filed Jan. 11, 1993, which issued as U.S. Pat. No.5,359,681 on Oct. 25, 1994.

TECHNICAL FIELD

The present invention is generally directed to a optical fiber sensorand, more specifically, to a optical fiber sensor which utilizes surfaceplasmon resonance to detect a sample, to a sensing apparatus whichemploys the optical fiber sensor, and to methods of detecting a sampleusing the same.

BACKGROUND OF THE INVENTION

Surface plasmon waves are electromagnetic waves which may exist at theboundary between a metal and a dielectric (hereinafter referred to asthe "sample"). Such waves can be exited by light which has its electricfield polarized parallel to the incident plane (i.e., transversemagnetic (TM) polarized). When the parallel component of the propagationconstant of the incident light equals the real part of the surfaceplasmon wave propagation constant, the incident light resonantly excitesthe surface plasmon waves, and a fraction of the incident light energyis transferred or dispersed to surface plasmon resonance (SPR). Thisdispersion of energy depends on both the dielectric constant of themetal and that of the sample in contact with the metal. By monitoringthe resonance wavevector of the metal/sample interface, the dielectricconstant of the sample (gas or solution) may be obtained. Alternatively,if the sample is contaminated by a chemical species, dielectric constantmeasurements may provide the concentration of the chemical species inthe sample.

Traditionally, SPR has been measured using the Kretschmann configuration(Kretschmann and Raether, Z. Naturforsch. Teil A 23:2135-2136, 1968). Inthis configuration, a thin layer of highly reflective metal (such asgold or silver) is deposited on the base of a prism. The metal surfaceis then contacted with the sample, and the SPR reflection spectra of thesample is measured by coupling TM polarized, monochromatic light intothe prism and measuring the reflected light intensity as a function ofthe angle of incidence. The angle of minimum reflective intensity is theresonance angle at which maximum coupling occurs between the incidentlight and the surface plasmon waves. This angle, as well as thehalf-width of the resonance spectrum and the intensity at the angle ofminimum reflective intensity, may be used to characterize or sense thesample which is in contact with the metal surface (Fontana et al.,Applied Optics 27:3334-3339, 1988).

Optical sensing systems have now been constructed based on theKretschmann configuration described above. Such systems utilize thesensitivity of SPR to changes in the refractive indices of both bulk andthin film samples, as well as to changes in the thickness of thin films.These systems, in conjunction with appropriate chemical sensing layers,have led to the development of a variety of SPR-based chemical sensors,including immunoassay sensors (e.g., Liedberg et al., Sensors andActuators 4:299-304, 1983; Daniels et al., Sensors and Actuators15:11-17, 1988; Jorgenson et al., IEEE/Engineering Medicine and BiologySociety Proceedings 12:440-442, 1990), gas sensors (e.g., Liedberg etal.,supra; Gent et al., Applied Optics 29:2843-2849, 1990), and liquidsensors (e.g., Matsubaru et al., Applied Optics 27:1160-1163, 1988).

While the Kretschmann configuration for SPR-based chemical sensorsoffers significant sensitivity, their relatively large size has severelyrestricted their application. For example, these bulk optic sensingsystems are limited by their use of a coupling prism causing suchsystems to be relatively large, expensive, and inapplicable for remotesensing applications. Moreover, such sensors generally require amonochromatic light source, are expensive to manufacture due toconfiguration constraints (such as the presence of a prism), and requirethat the incident light sweep over a broad range of incidence angles.

Accordingly, there is a need in the art for an improved SPR sensor, aswell as for apparatus and methods relating thereto. Specifically, thereis the need for an SPR sensor which readily permits remote sensing, isinexpensive, and is free from the limiting constraints now present withexisting SPR-based chemical sensors.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a optical fibersensor which utilizes SPR to detect a sample in contact with the sensor.It is a further object to provide a sensor which utilizes an opticalfiber as the sensor itself, and avoids the use of a coupling prism. Anadditional object is to provide a fiber optic SPR sensor which utilizesincident light having multiple wavelengths as the excitation energy. Yeta further object is to provide methods and apparatus for detecting asample using the fiber optic SPR sensor of this invention. The presentinvention fulfills these objectives, and provides further relatedadvantages.

In one embodiment of this invention, a fiber optic SPR sensing apparatusis disclosed. The apparatus contains a fiber optic SPR sensor incombination with a source of electromagnetic radiation of multiplewavelengths and a detection device. The fiber optic SPR sensor is anoptical fiber having a core waveguide and a cladding or cladding/bufferlayer surrounding the core waveguide, and wherein the optical fiber hasa first end and a second end and at least one sensing area locatedbetween the first end and the second end or located at the second end ofthe optical fiber. The sensing area of the optical fiber is defined byan SPR supporting metal layer which is contact with at least a portionof an exposed surface of the optical fiber core waveguide which is freefrom the surrounding cladding or cladding/buffer layer. The output fromthe electromagnetic radiation source is applied to the first end of theoptical fiber core waveguide such that the radiation propagates from thefirst end towards the second end by total internal reflections (TIR),and exits the optical fiber at either the first end or the second end. Adetection device monitors the radiation exiting an end of the opticalfiber.

In a preferred embodiment, the fiber optic SPR sensor is an in-linetransmission-based optical fiber sensor. Such a sensor contains anoptical fiber having a core waveguide and a cladding or cladding/bufferlayer surrounding the core waveguide, and having an input end and anoutput end. The sensor has a sensing area located between the input endand output end defined by an SPR supporting metal layer in contact withat least a portion of the optical fiber core waveguide free from thesurrounding cladding or cladding/buffer layer. A source ofelectromagnetic radiation of multiple wavelengths is applied to theinput end of the optical fiber core waveguide such that the radiationpropagates from the input end towards the output end by TIR. A detectiondevice then monitors the radiation exiting the output end of the opticalfiber waveguide.

In a further preferred embodiment, the sensing apparatus of the presentincludes a terminated reflection-based fiber optic SPR sensor. Thissensor contains an optical fiber having a core waveguide and a claddingor cladding/buffer layer surrounding the core waveguide, and having aninput/output end and a terminal reflection end. The terminal reflectionend is defined by an end face of the core waveguide in contact with amirrored layer. The sensing area of the sensor is located between theinput/output end and terminal reflection end, and is defined by an SPRsupporting metal layer in contact with at least a portion of the opticalfiber core waveguide free from the surrounding cladding orcladding/buffer layer. A source of electromagnetic radiation of multiplewavelengths is applied to the input/output end of the optical fiberwaveguide, and the radiation propagates from the input/output endtowards the terminal reflection end by total internal reflections,internally reflects off the mirrored layer in contact with the end faceof the core waveguide, and propagates back down the optical fiberwaveguide by total internal reflections towards the input/output end. Adetection device monitors the radiation exiting the input/output end ofthe optical fiber waveguide.

In another embodiment of this invention, a method for detecting a sampleis disclosed. In this method, a sample was contacted with a fiber opticSPR sensor of the present invention. A source of electromagneticradiation of multiple wavelengths is applied to one end of the fiberoptic SPR sensor and the radiation exiting the sensor is detected.

In yet a further embodiment, the present invention discloses a fiberoptic SPR sensor. The sensor has an optical fiber core waveguide and acladding or cladding/buffer layer surrounding the core waveguide. Asensing area is located between a first and second end of the opticalfiber and is defined by an SPR supporting metal layer in contact with atleast a portion of the surface of the optical fiber core waveguide freefrom the surrounding cladding or cladding/buffer layer. In preferredembodiments, the fiber optic SPR sensor is an in-line transmission-basedfiber optic SPR sensor or a terminated reflection-based fiber optic SPRsensor.

These and other aspects of this invention will become evident uponreference to the following detailed description and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) represents the prior art Kretschmann configuration for a bulkoptic SPR-based chemical sensor, and FIG. 1(b) illustrates an SPRreflection spectra obtained from the prior art sensor of FIG. 1(a) for abulk dielectric of water, a wavelength of 620 nm, a 550 Angstrom-thicksilver film, and a fused silica prism.

FIG. 2 illustrates an in-line transmission-based fiber optic SPR sensorof this invention.

FIG. 3 illustrates a terminated reflection-based fiber optic SPR sensor.

FIG. 4 illustrates a further embodiment of a terminated reflection-basedfiber optic SPR sensor.

FIG. 5 is a calculated three-dimensional SPR reflection spectra for abulk dielectric of water.

FIG. 6 is a contour plot of two theoretical three-dimensional SPRreflection spectra for water and a 37.1% sucrose solution.

FIG. 7 is the calculated SPR spectra of reflected light intensity versuswavelength for a number of angles propagating inside a optical fibersensor.

FIG. 8 is a calculated SPR spectra taking into account the number ofreflections each angle encounters.

FIG. 9 is the non-linear distribution function of propagating angleswithin the optical fiber, and assumed to be Gaussian.

FIG. 10 is the calculated SPR spectra for a bulk refractive index ofwater and a 37.1% sucrose solution.

FIG. 11 illustrates a sensing apparatus of the present inventionemploying an in-line transmission-based fiber optic SPR sensor.

FIG. 12(a) illustrates representative air and sample SPR spectra, FIG.12(b) depicts a normalized sample SPR spectra, and FIG. 12(c) arenormalized SPR spectra collected using the in-line transmission-basedfiber optic SPR sensor of Example 2.

FIG. 13 is a plot of the normalized SPR optical fiber sensor spectrameasured by the 6, 10, and 18 mm in-line fiber optic SPR sensors ofExample 2.

FIG. 14 depicts the calculated and experimentally determined SPRcoupling wavelength versus the refractive index for the sucrosesolutions of Example 2.

FIG. 15 illustrates a sensing apparatus of the present inventionutilizing a terminated reflection-based fiber optic SPR sensor.

FIG. 16 illustrates the theoretical and experimentally observed SPRcoupling wavelength versus the refractive index of the solutionsdetected in Example 3 utilizing an SPR supporting metal layer of eithersilver or gold.

FIG. 17 illustrates the normalized SPR reflection spectra (referenced toair) for Buffer A, de-ionized water, BSA and rhFXIII.

FIG. 18 illustrates the shift in the normalized SPR spectra due tobinding of rb-antiFXIII over time (0-55 minutes).

FIG. 19 illustrates the shift in the SPR spectra of FIG. 18 over time.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a optical fiber sensor which detects asample in contact with the sensor by surface plasmon resonance (SPR)measurements. The present invention is also directed to methods andapparatus relating to the use of the fiber optic SPR sensor to detect asample. In preferred embodiments, the fiber optic SPR sensor of thisinvention includes an in-line transmission-based sensor and a terminatedreflection-based sensor. The fiber optic SPR sensor of this inventionadvantageously eliminates the traditional bulk optic prism in favor ofan optical fiber design which permits remote sensing and multiplexingbetween multiple fiber optic SPR sensors.

As mentioned above, prior art SPR-based chemical sensors are generallybased on the Kretschmann configuration (see, Kretschmann and Raether, Z.Naturforsch. Teil A 23:2135-2136, 1968). Such a prior art SPR-basedchemical sensor is illustrated in FIG. 1(a). Specifically, a highlyreflective metal layer 10), such as gold or silver, is deposited on base(12) of prism (14). TM polarized, monochromatic incident light (16) isdirected into the prism and reflects off the prism base/metal layerinterface. The intensity of reflected light (18) is measured by adetection device (not shown). A sample (11) is brought in contact withexposed surface (15) of metal layer (10), and the monochromatic incidentlight is directed into the prism at angle θ with respect to the normalof the metal layer/sample interface. At appropriate angles of incidence,the monochromatic incident light excites surface plasmon waves (13). TheSPR reflection spectra obtained from the prior art SPR-based chemicalsensor of FIG. 1(a) is represented in FIG. 1(b) by a two-dimensionalplot of reflected light intensity versus the angle of incidence. Theangle of minimum reflective intensity is the surface plasmon resonanceangle (θ_(sp)) at which maximum coupling of energy occurs between theincident light and the surface plasmon waves. This angle is dependentupon the sample in contact with the exposed surface of the metal layer.

In contrast to prior art SPR-based chemical sensors which utilize asingle monochromatic incident light source and modulate the incidentangle, the fiber optic SPR sensor of this invention employs a limitedrange of incident angles and uses incident light having multiplewavelengths. As used herein, the phrase "incident light having multiplewavelengths" means, at a minimum, two wavelengths, and is preferable arange of wavelengths sufficiently broad to encompass the resonancespectrum of the sample. For example, black body radiation or a whitelight source may serve as the incident light. Alternatively, two or morediscrete wavelengths may be employed in the practice of the presentinvention. Furthermore, as used herein, the phrase "limited range ofincident angles" means the range of propagating angles supported by agiven optical fiber.

Suitable optical fibers of the present invention include commerciallyavailable fibers which support internal propagation of light by TIR.Such fibers may generally be characterized by three parameters: theoptical fiber core material, the numerical aperture of the fiber, andthe optical fiber core diameter. The choice of the optical fiber corematerial will effect the position of the resonance (i.e., where itoccurs in the wavelength region). For example, a silica optical fiberwith a refractive index of 1.46 permits measurements of effectiverefractive indices from about 1.32 to about 1.45, and may beexperimentally observed as the resonance shifts from 400 nm up to 1000nm. Other optical fiber core materials will similarly effect theposition of the resonance. Thus, use of optical fiber core materials ofa higher refractive index than silica will shift the dynamic range ofthe effective refractive indices to higher values. For example,sapphire, with a refractive index of 1.76, will result in a sensor witha dynamic range of effective refractive indices of from about 1.45 toabout 1.75. Similarly, use of a plastic optical fiber core, such as apolymethylmethacrylite (PMMA) with a refractive index of 1.50, wouldpermit measurements of effective refractive indices from about 1.33 toabout 1.49. If an optical fiber core material with a lower refractiveindex than silica was employed, the dynamic range of the measurablesample refractive index would be shifted towards smaller values. Thedynamic range of the fiber optic SPR sensors may also be modified byaddition of appropriate dynamic range-controlling layers, as discussedin greater detail below.

The numerical aperture of the optical fiber determines the acceptanceangle of light that is allowed to propagate in the fiber. In thepractice of this invention, suitable numerical apertures may range fromabout 0.05 to about 0.6, and are preferably from about 0.2 to 0.4, andmost preferably from about 0.25 to about 0.35. The numerical aperturealso determines the internal propagation angles of the light propagatingin the fiber. For example, a silica optical fiber with a numericalaperture of 0.3 supports internal propagating angles of light rangingfrom 90° to 78.5° (relative to the normal of the core/claddinginterface), and thus the limited range of incident angles for this fiberwould range from 90° to 78.5°. The core diameter of the fiber may varydepending upon the specific application. Preferably, the optical fibercore diameter ranges from about 1 micron to about 2000 microns, and morepreferably from 100 microns to 600 microns, and most preferably from 200microns to 400 microns.

As an optional component, one end of the optical fiber may have anelement which reflects the light propagating within the fiber such thatthe light reverses its direction of propagation. A suitable elementwhich may accomplish this function is a mirrored layer located on theend surface of the optical fiber core. For example, if light is directedinto the optical fiber at a first end, and the mirrored layer is locatedat a second end of the fiber, light propagating toward the second endwill be reflected (upon contact with the mirrored layer) back toward thefirst end of the optical fiber. Such mirrored layers may be adhered tothe end surface of the optical fiber core by known techniques, such asby electron beam evaporation, thermal evaporation, sputtering,electrodeless plating, or adhering or gluing a suitable mirrored layerto the end of the fiber. Suitable materials for the mirrored layerincluding highly reflective metals, such as silver, gold and chrome. Themirrored layer should be of sufficient thickness to provide adequatereflectivity, and should not support SPR at the end of the fiber. Ametal thickness of about 2000 Angstroms (or greater) generally providessufficient reflectivity, and a metal thickness in excess of about 1000Angstroms generally will not support SPR at the end of the fiber. Theappropriate thickness for any given mirrored layer having the abovecharacteristics may be readily determined by one skilled in this art.

Detection of a sample with the fiber optic SPR sensor of this inventionis made, in part, by contacting the sample with the sensing area of theoptical fiber. The sensing area is made by exposing a portion of theoptical fiber core by removal of the surrounding cladding orcladding/buffer layers, and adhering an SPR supporting metal layer tothe exposed optical fiber core. The SPR supporting metal layer of theoptical fiber is then exposed to the sample of interest, and therefractive index of the sample is determined by the methods disclosedbelow.

The cladding or cladding/buffer layers of the optical fiber may beremoved to expose a portion of the core by know techniques. For example,the cladding or cladding/buffer layers may be removed by a torch, or bychemical agents which etch away the cladding or cladding/buffer layerswhile preserving the fiber core material. Alternatively, the cladding orcladding/buffer layers may be removed by commercially available,mechanical strippers (e.g., Clausse, No-Nik Optical fiber Stripper,Edmond Scientific Catalog, Barrington, N.J.).

Once a portion of the optical fiber core is exposed, the SPR supportingmetal layer is adhered to the exposed core. As used herein, the term"SPR supporting metal layer" means a highly reflective metal thatsupports SPR at the metal/sample interface, and has a permittivityconstant wherein the real part of the permittivity is negative and itsmagnitude is greater than the magnitude of the imaginary part. Withinthe visible and near-IR region (400 nm-1000 nm), both silver and goldsatisfy this criteria. However, if the above wavelength range isextended into the infrared, other metals, such as aluminum, copper andtantalum, may also be employed.

The SPR supporting metal layer is preferably adhered to the exposedportion of the optical fiber core to a thickness which will optimize theresonance curve--that is, to a thickness which makes the SPR resonancespectrum sharp. When the SPR supporting metal layer is silver, thislayer is preferably adhered to the exposed core at a thickness of about550 Angstroms. If a thinner thickness is used, the resonance spectrawill substantially broaden, and if a thickness in excess of 600Angstroms is employed, the resonance will severely diminish ordisappear. One skilled in this art may readily determine the appropriatethickness of the SPR supporting metal layer for any given opticalfiber/SPR supporting metal layer combination by varying the thickness tooptimize the resonance curve.

A single optical fiber may contain one or more sensing areas, of thesame or different geometry, and with the same or different SPRsupporting metal layers. Such sensing areas may be located along thelength of the optical fiber, at one end of the optical fiber, or both.In addition, while any portion of the optical fiber may serve as thesensing area, in a preferred embodiment the cladding or cladding/bufferlayers are removed from the entire circumference of the optical fibercore, and the SPR supporting metal layer is symmetrically deposited onthe exposed core to a uniform thickness.

An energy source which emits light having multiple wavelengths serves asthe source of incident radiation. Generation of the incident radiationmay be by any of a number of commercially available devices. Forexample, a tungsten halogen lamp provides radiation with a wavelengthrange that is sufficiently broad to encompass the resonance spectrumbetween 400 nm and 1000 nm. However, the other white light sources canbe employed. Moreover, best results may obtained when the current andtemperature of a white light source are controlled in order to minimizeany background spectral variation. The energy source may be coupled intothe optical fiber by use of commercially available optical fiberillumination instruments, such as an Oriel, Optical fiber Source forRadiometry (this specific instrument focuses white light emitted from abulb into one end of the optical fiber).

Suitable detection devices of the present invention are capable ofdetecting the intensity of all or a portion of the wavelengths of lightexiting the optical fiber. For example, when the fiber is connected to aoptical fiber spectrograph, the light exiting the fiber is reflected offa grating towards a linear array detector. Upon reflectance off thegrating, the light is linearly dispersed as a function of wavelength.Individual photodiodes in the array detector then measure the intensityalong the length of the array detector, and detects the light intensityversus linear displacement (which is proportional to wavelength). Aspectrophotometer may also be employed to measure light intensity versuswavelength, or a circular variable interference filter wheel may be usedin front of a photodetector. Such a filter wheel allows for a certainnarrow bandpass of light which changes as a function of wheel rotation,and permits measurement of the spectral intensity of the light. Similardetection devices could employ a dispersing prism, linear variableinterface filter, or individual interference filters when only a limitednumber of wavelengths are of interest.

By measuring the resonance spectrum, the complex refractive index of thesample in contact with the sensing area of the optical fiber sensor canbe determined. A sample's complex refractive index includes both thereal and imaginary refractive index components. The real component of asample's complex refractive index is inversely proportional to the speedat which light propagates through the sample, and is generallyconsidered the "true"refractive index of the sample. The imaginarycomponent of a sample's complex refractive index is related to thesample's absorbance or attenuation of light. For example, by measuringthe resonance spectrum of a solution containing sugar, the concentrationof the sugar can be determined (assuming the sugar is the only varyinganalyte in the solution that caused the real refractive index of thesample to change). Such measurements have utility in the manufacture ofsugar-containing beverages, as well as the alcohol content of asolution, or the hydrogenation of vegetable oil. Similarly, the fiberoptic SPR sensor of this invention may be used to measure the absorbanceof a sample. For example, use of a dye indicator for one or morespecific analytes within a sample (such as acid/base dye indicators forpH and CO₂) may be employed.

In addition, the sensing area may optionally contain one or moreadditional layers adhered to the SPR supporting metal layer to yieldeffective refractive indices detectable by the sensor. Such additionallayers may include a dynamic range-controlling layer and/or a reactivelayer. A "dynamic range-controlling layer" is a layer adhered to the SPRsupporting metal layer to alter the dynamic range of the fiber optic SPRsensor. For example, the dynamic range for a silica fiber, having arefractive index of 1.46, is about 1.32 to about 1.45. Adherence of adynamic range-controlling layer of low refractive index (e.g., 1.2) tothe SPR supporting metal layer will shift the dynamic range of thesensor to higher values (e.g., 1.42 to 1.55).

As used herein, the term "reactive layer" means a layer which interactswith the sample such that the effective refractive index detected by thesensor is altered. The addition of a reactive layer permits themanufacture of a fiber optic SPR sensor which is more sensitive to, ormore selective for, a sample (or analyte within a sample). For example,suitable reactive layers include an antigen or antibody bound to the SPRsupporting metal layer. This type of reactive layer will selectivelybind the complementary antibody or antigen in the sample, increase thethickness of the reactive layer, and causes a shift in the effectiverefractive index measured by the sensor. In general, suitable reactivelayers are altered in some manner upon contact with the sample, thuschanging the effective refractive index measured by the sensor. Otherreactive layers include sol-gel films and polymer coatings, and may beadhered to the SPR supporting metal layer by known techniques.

FIGS. 2-4 illustrate preferred embodiments of the fiber optic SPR sensorof this invention. In FIG. 2, an in-line transmission-based fiber opticSPR sensor is depicted. This sensor is made by removing a section ofoptical fiber cladding (21) and buffer (23) from core (26) of opticalfiber (20), and depositing an SPR supporting metal layer (26) on exposedsurface (28) of core (22).

In FIG. 3, a terminated reflection-based fiber optic SPR sensor isillustrated. In this embodiment, a mirrored layer (34) is adhered to theend of optical fiber (30). Specifically, the mirrored layer is incontact with end face (32) of optical fiber core (36), and coverscladding layer (37) and buffer layer (38) of the optical fiber. An SPRsupporting metal layer (39) is adhered to an exposed surface (33) ofcore (36).

FIG. 4 illustrates a further embodiment of a terminated reflection-basedfiber optic SPR sensor wherein the sensing area is located at the end ofthe optical fiber. Cladding layer (42) and buffer layer (44) are removedfrom optical fiber (40) to expose surface (45) of core (46). An SPRsupporting metal layer (48) is adhered to exposed surface (45) of core(46). Mirrored layer (49) is in contact with end face (43) of core (46).

The following examples are offered by way of illustration, notlimitation.

EXAMPLES Example 1 Calculated SPR Shift Between Water and SucroseSolution

A calculated three-dimensional SPR reflection spectra for a range ofwavelengths (400-1000 nm) is illustrated in FIG. 5. This spectra wascalculated using a matrix method to determine the fresnel reflectioncoefficients of a multilayered structure and assumes a silica opticalfiber, a 550 Angstrom silver layer, a bulk sample of water, and TMpolarized light. The refractive index values for silver silica and waterwas obtained from the literature (e.g., see Hass and Hadley, OpticalProperties of Metals, American Institute of Physics Handbook, D. Grayed., McGraw-Hill, New York, pp. 149-151, 1972; Querry et al., Water (H₂O), Handbook of Optical Constants of Solids II, E. Palik ed., AmericanPress, Boston, pp. 1059-1077, 1991; Malitson, J.O.S.A. 55:1205-1216,1965). The change in the SPR coupling angle as a function of wavelength(as illustrated in FIG. 5) is primarily due to the increased magnitudeof the silver complex refractive index over the wavelength range of400-1000 nm. This large change in silver refractive index (about oneorder of magnitude) is compared to the relatively small change in boththe silica and water refractive indices (i.e., 0.019 and 0.128 index ofrefraction units, respectively). FIG. 6 depicts a contour plot of twocalculated three-dimensional SPR reflection spectra for water and asucrose solution (37.1% by weight) as the bulk sample media. This figureillustrates the resonance coupling angle dependency upon wavelength, andthe three-dimensional SPR spectra dependence upon the bulk dielectricrefractive index between the water and sucrose spectra.

In order to model the spectrum of the SPR fiber optic sensors severalfactors must be considered, including: (1) the three dimensional SPRreflection spectra, (2) the number of reflections each propagating modeof undergoes, and (3) the density of propagating modes in the opticalfiber sensor. FIG. 7 illustrates the theoretical SPR spectra (assuming acore diameter of 400 microns, a sensing area of 10 mm in length, and aconstant light intensity over all wavelengths) of reflected intensityfor one reflection versus wavelength for a number discrete anglespropagating inside the fiber optical sensor (i.e., 90°, 87°, 84°, 81°and 78°). The number of reflections in the fiber sensor area, N, is afunction of the mode propagation angle, θ, the diameter of the fibercore, d, and the length of the sensing area, L, and is governed by thefollowing equation:

    N=L/d tan θ

Thus, to determine the effective SPR spectra, taking into account themultiple reflections, the spectra for a single reflection is raised tothe power of the number of reflections the specific propagating angleundergoes with the sensor interface. FIG. 8 illustrates the effectiveSPR spectra of intensity versus the wavelength for the propagatingangles of FIG. 7. The spectra for the lowest order mode, 90°), travelsparallel to the meridonial axis of the fiber and does not reflect offthe interface. Therefore, the spectrum is that of the 90° spectrum ofFIG. 6 raised to the power of zero, which yields a constant spectrumcorresponding to no surface plasmon waves excited by the 90° propagatingangle. Similarly, the smallest propagating angle in the fiber (i.e.,78°) has an effective spectra which is greatly broadened since itundergoes 5.31 reflections within the length of the sensing area.

The SPR fiber optic signal detected at the output end of the fiberrepresents an accumulated spectra for the entire range of propagatingangles, and not the spectra for any specific mode. Moreover, this signalis not an equally weighted average of all the angles propagating in thefiber. Thus, the theoretical signal must be weighted with the energydistribution function of all propagating angles in the fiber. For thispurpose, the propagation angle density distribution function may beassumed to be Gaussian, spanning the range of the allowed propagatingangles in the fiber is illustrated in FIG. 9. FIG. 10 represents thetheoretical SPR fiber optic signal obtained by weight averaging theangular spectra of FIG. 8 with the density distribution of function ofFIG. 9 for a bulk water sample and a 37.1% sucrose solution. Thus, anapproximate 200 nm shift in SPR coupling wavelength is predicted, andcorresponds to a change of 0.06 index of refraction units between thewater and sucrose samples.

Example 2 In-Line Transmission-Based Fiber optic SPR Sensor

A silica/polymer fiber Type FP-400 UHT (3M, Minneapolis, Minn.), havinga diameter of 400/600/760 microns (i.e., core, cladding and bufferdiameters, respectively) and a numerical aperture of 0.3 was used inthis example. The buffer and cladding layers were removed by a torch(Weber and Schultz, Biosensors and Bioeletronics 7:1930197, 1992), andthe exposed core surface wiped with Dynasolve 100 (Dynaloy Inc.,Hanover, N.J.). Specifically, three fiber optic sensors were fabricatedby removing 6, 10 and 18 mm of the cladding/buffer layers along thelength of three fibers. Each fiber was then mounted in an electron-beamevaporator such that the flux of the evaporated metal (silver) wasperpendicular to the axis of the fiber. The fibers were rotated duringsilver deposition, resulting in a 550 Angstrom silver film depositedsymmetrically about the fiber. The deposition process was monitoredusing a quartz crystal detector.

FIG. 11 illustrates the experimental set-up of this example. The threeinline transmission-based fiber optic SPR sensors were fabricated asdisclosed above. The output of a tungsten halogen lamp was independentlyfocused into each of the optical fibers. A mode scrambler was used topopulate all modes of the optical fibers. The sensing area of the sensorwas then enclosed by a three milliliter flow cell constructed using asyringe with two syringe stoppers and inlet and outlet ports. The outputof the fiber optic SPR sensor was connected to a fiber opticspectrograph (American Holographic, Littleton, Mass.) via a SMAconnector. The flat field grating is an American Holographic Model#446.33, which dispersed a range of wavelengths from 400 nm to 900 nmwith a linear dispersion value of 20 nm/mm. The detector inside thespectrograph was a 1024-element CCD linear array detector. Thetheoretical wavelength resolution was determined by the lineardispersion value of the grating and the 25.4 micron width of the CCDelement is 0.5 nm. A data acquisition board was used with an IBMcompatible computer for automated acquisition.

Six sample solutions of high fructose corn syrup diluted with deionizedwater were prepared. The refractive indexes of these samples solutionswere determined to be 1.333, 1,351, 1.364, 1.381, 1.393 and 1.404,respectively, using an Abbe Refractometer (Milton Roy TabletopRefractometer 3L) at a 589 nm wavelength. The transmitted spectralintensity distribution was measured for each sensor while air was in theflow cell, and then measured again for each of the six prepared sucrosesolutions by introducing 15 ml of each solution into the input port ofthe flow cell.

Since the intensity of the incident light was not constant for allwavelengths (400-900 nm), an air spectra was collected when the sensingarea of the sensor was in contact with air (i.e., there is no surfaceplasmon resonance excitation in this wavelength range for air having abulk refractive index of 1.00). The SPR spectra collected for eachsample was normalized against the air spectra using the followingequation:

    N(λ)=1- I.sub.air (λ)-I.sub.sample (λ))/I.sub.air (λ)!

where I_(air) (λ) is the intensity of the air spectra at wavelength λand I_(sample) (λ) is the intensity of the sample spectra at wavelengthλ . A plot of N(λ) versus λ yields the normalized spectra. FIG. 12(a)illustrates a representative air and sample SPR spectra, and FIG. 12(b)depicts the normalized sample spectra according to the above equation.This calibration technique effectively normalizes the system transferfunction, attributed by the light spectral output, the photo diode arrayspectral sensitivity, and the fiber spectral absorbance. FIG. 12(c)depicts the normalized transmitted light intensity as a function ofwavelength measured by the 10 mm sensor for the fructose solutions withrefractive indexes of 1.351, 1,393 and 1,404.

The resonance wavelength shift for the increasing bulk refractiveindices of the sample is consistent with the calculated resultsillustrated in FIG. 10. However, it should be noted that the resonancespectra appears slightly broader than predicted. This observation isbelieved to be attributable, in least in part, to the large input fiberconnected to the spectrograph (i.e., 400 microns), and thus the fullspectrograph resolution is not optimized. It is believed that narrowerresonance spectra may be achieved by reducing the diameter of the inputoptical fiber core to the spectrograph or by employing the use of anarrower slit.

FIG. 13 is a plot of the SPR optical fiber sensor spectra measured bythe 6, 10 and 18 mm sensors for a single fructose solution (i.e.,refractive index of 1.351). The transmitted spectral intensitydistribution depends upon the length of the SPR sensing area asillustrated in FIG. 13, a longer sensing area resulted in a deeperobserved resonance spectra. Thus, the size of the sensing area can beoptimized, and primarily depends on the surface area of the sensingarea, the diameter of the optical fiber core, and the numericalaperture. Due to the cylindrical geometry of the fiber, both TE and TMpolarized light (with respect to the core/metal interface) are allowedto propagate in this multi-mode fiber. Thus, the expected optimaltransmitted light intensity at resonance is 0.5 rather than 0.0, sinceSPR can only be excited with TM polarized light.

Theoretical and experimental SPR coupling wavelength versus therefractive index of the fructose solutions are plotted in FIG. 14 forthe three SPR optical fiber sensors of FIG. 13. The response of allthree sensors are in good agreement with the calculated shifts ofExample 1. To plot the Abbe Refractometer measured refractive indices ofthe sample solutions in FIG. 14 the values were corrected for wavelengthusing the refractive dispersion measurement (Abbe RefractometerOperator's Manual and Dispersion Table, Milton Roy Company, AnalyticalProducts Division, Rochester, N.Y., 1986). The theoretical sensitivityof the fiber optic SPR sensor to refractive index was calculated fromthe SPR wavelength response curve of FIG. 13. Because the response isnon-linear, the sensitivity is a function of wavelength, with increasedsensitivity at longer wavelengths. The theoretical sensitivity torefractive indices is 2.5×10⁻⁴ at a wavelength of 500 nm, and 7.5×10⁻⁵at a wavelength of 900 nm, assuming an optimal wavelength resolution ofthe spectrograph of 0.5 nm, and the observed sensitivity at thesewavelengths is in good agreement.

Example 3 Terminated Reflection-Based Fiber optic SPR Sensor

FIG. 15 illustrates a terminated reflection-based fiber optic SPR sensorof the present invention. This sensor is similar to the in-linetransmission sensor described above, but uses a micro-fabricated mirrorat the end of the optical fiber to internally reflect the lightpropagating through the fiber. In this embodiment, the light travelsthrough the sensing area twice, thus the sensing length can be one halfthe length of the sensing area of the in-line transmission sensor.

The terminated reflection-based fiber optic SPR sensors were constructedby stripping off 1 cm of the cladding and buffer layers at the end oftwo separate silica optical fibers (0.3 numerical aperture, 400 microncore diameter). The fibers were then individually mounted vertically inan electron beam evaporator chamber such that the stripped end of eachfiber was face down. An evaporated metal (silver) was then deposited onthe stripped end face of each fiber, to a thickness of 3000 Angstroms byusing a mask to prevent deposition on the exposed surface of the core.The fibers were then mounted in an electron beam chamber in the samefashion as the in-line fiber-optic sensors of Example 2, and either asilver or gold metal layer symmetrically deposited on the exposed coresurface of the fibers to a thickness of 550 Angstroms.

In this example, light was coupled into a single branch of a 50:50 twoway fiber optic splitter from a tungsten-halogen lamp. Fifty percent ofthe coupled light was then transferred to the sensor branch of thesplitter. A SMA connector was used to connect the splitter to the SPRfiber optic sensor. The light was transmitted down the probe to thesensor area and reflected back up the optical fiber by themicro-fabricated mirror. The mirror thickness was such that SPR does notoccur at the end of the optical fiber (in this example, the mirrorthickness is 3000 Angstroms). The returned light is then split again andconnected to an optical fiber spectrograph to measure the spectralintensity of the signal light. The remaining arm of the splitter wasindex matched to a solution of glycerol so as to minimize backreflection. However, a reference signal could also be measured by usinga spectrograph with two inputs, one for the signal and the other of areal time reference. The liquid samples of this experiment was sixsolutions containing glycerol at various concentrations to yield sampleshaving different refractive indices.

The results obtained from the two terminated probes are illustrated inFIG. 16, and are consistent with the data and calculated results of anin-line transmission SPR fiber optic sensors. The wavelength offsetobserved between the terminated gold and silver probes is believed to bedue to the difference in the permittivity constants between the twometals (see FIG. 16) of the SPR supporting metal layer.

Example 4 Immunoassay Utilizing Terminated Reflection-Based Fiber OpticSPR Sensor

This example illustrates the use of the fiber optic SPR sensor ofExample 3 in combination with a reactive layer deposited on the surfaceof a SPR supporting gold layer. The observed shifts in the SPR spectraof Example 3 above (ie., FIG. 16) towards larger coupling wavelengths,λ, is due to increasing refractive indices of the bulk chemical sample.A similar shift can be caused by the presence of a thin film reactivelayer between the metal and the sample. The adsorption of such a thinfilm layer (provided the thin film layer has a different refractiveindex than the bulk sample) will cause a shift in λ. This is because thesurface plasmon waves "sense" an effective refractive index, n_(eff),that is representative of the combination of the refractive indices ofthe surface plasmon support layer, the thin film, and the thickness ofthe thin film. Changes in the film parameters (such as film thicknessand/or complex refractive index) will cause a shift in λ_(spr). Thesechanges can be solved for by analyzing the properties of the measuredSPR spectrum.

To illustrate this aspect of the present invention, protein solutionswere prepared containing the following components: recombinant humanFactor XIII (rhFXIII) expressed and purified from yeast (ZymoGenetics,Inc., Seattle, Wash.), bovine serum albumin (BSA, Fraction V, Sigma, St.Louis, Mo.), and a rabbit anti-rhFXIII biotinylated polyclonal IgGpreparation (rb-antiFXMIII, Protein-A purified, est. 98% IgG). Both therhFXIII and BSA samples were at a concentration of 10 mg/ml in Buffer Asolution (pH 7,2, 2% sucrose, 0.1 mM EDTA-Na₂, 10 mM glycine) while therb-antiFXIII was at 0.5 mg/ml in phosphate buffered saline (pH 7.4). Allchemicals were purchased from Sigma Chemical.

FIG. 17 illustrates the SPR reflection spectra, referenced to air, forBuffer A, deionized water, BSA and rhFXIII which was obtained by dippingthe terminated reflection-based fiber optic SPR sensor in the respectivesolution. The spectra was not effected by mechanical agitation orstirring of the test solution and the spectra for both the BSA andrhFXIII solutions shifted to the same extent. This indicates that thesensor is insensitive to the molecular weight of the protein (rhFXIII is168 kD and BSA is 69 kD), but responsive to the mass of protein adsorbedto the surface of the sensing area.

The sensor was referenced to air, and tested in Buffer A and deionizedwater. The sensor was then coated with rhFXIII by placing the sensor inthe rhFXIII solution until no spectral shift occurred. The sensor wasthen dipped into the BSA solution, and the interaction of therb-antiFXIII was studied over a course of 55 minutes by placing thesensor in the antibody solution. The interaction of the polyclonalrb-antiFXIII with rhFXIII absorbed to the surface of the SPR supportingmetal layer was found to be sufficient slow (hours) to permitmeasurement. The temporal shift in SPR spectra due to binding ofrb-antiFXIII is illustrated in FIG. 18, and determination of the minimafor each SPR spectra is depicted in FIG. 19.

As illustrated by FIG. 19, over 50% of the observed response occurredwithin the first 10 minutes of exposure. The multi-phasic responsebehavior of the sensor is consistent with the nature of the polyclonalantibody use in this study, and represents the average response from amultitude of anti-ideotype IgGs with differing kinetic characteristicsfor rhFXIII. The sensor response was found to be reversible, asdemonstrated by cleaning the probe with a 0.1N NaOH solution, and bysubsequent spectral comparisons (for air and Buffer A) between thesensor prior to exposure to the test solutions versus subsequent toexposure.

From the foregoing, it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

We claim:
 1. An in-line transmission-based optical fiber surface plasmonresonance sensor, comprising an optical fiber having a core and acladding or cladding/buffer layer surrounding the core, and having aninput end and an output end, wherein the optical fiber has a sensingarea located between the input end and output end, and wherein thesensing area is defined by a layer of surface plasmon resonancesupporting metal in contact with at least a portion of the optical fibercore free from the surrounding cladding or cladding/buffer layer.
 2. Aterminated reflection-based optical fiber surface plasmon resonancesensor, comprising an optical fiber having a core and a cladding orcladding/buffer layer surrounding the core, and having an input/outputend and a terminal reflection end, wherein the terminal reflection endis defined by an end face of the core in contact with a reflective layerthat does not support surface plasmon resonance, wherein the opticalfiber has a sensing area located between the input/output end andterminal reflection end or at the terminal reflection end, and whereinthe sensing area is defined by a layer of surface plasmon resonancesupporting metal in contact with at least a portion of the optical fibercore free from the surrounding cladding or cladding/buffer layer.
 3. Theoptical fiber surface plasmon resonance sensor of claim 1 or 2 whereinthe sensing area contains at least one additional layer in contact withthe layer of surface plasmon resonance supporting metal.
 4. The opticalfiber surface plasmon resonance sensor of claim 3 wherein at least oneadditional layer comprises a reactive layer.
 5. A fiber optic surfaceplasmon resonance system, comprising:a) an in-line transmission-basedfiber optic surface plasmon resonance sensor wherein the sensorcomprises an optical fiber having a core and a cladding orcladding/buffer layer surrounding the core, and having an input end andan output end, wherein the optical fiber has a sensing area locatedbetween the input end and output end, and wherein the sensing area isdefined by a layer of surface plasmon resonance supporting metal incontact with at least a portion of the optical fiber core free from thesurrounding cladding or cladding/buffer layer; b) a source ofelectromagnetic radiation of multiple wavelengths whose output isapplied to the input end of the optical fiber such that the radiationpropagates from the input end towards the output end by total internalreflections; and c) a detector which receives the radiation exiting theoutput end of the optical fiber.
 6. A fiber optic surface plasmonresonance system, comprising:a) a terminated reflection-based fiberoptic surface plasmon resonance sensor wherein the sensor comprises anoptical fiber having a core and a cladding or cladding/buffer layersurrounding the core, and having an input/output end and a terminalreflection end, wherein the terminal reflection end is defined by an endface of the core in contact with a reflective layer that does notsupport surface plasmon resonance, wherein the optical fiber has asensing area located between the input/output end and the terminalreflection end or at the terminal reflection end, and wherein thesensing area is defined by a layer of surface plasmon resonancesupporting metal in contact with at least a portion of the optical fibercore free from the surrounding cladding or cladding/buffer layer; b) asource of electromagnetic radiation of multiple wavelengths whose outputis applied to the input/output end of the optical fiber, and wherein theradiation propagates from the input/output end towards the terminalreflection end by total internal reflections, internally reflects offthe reflective layer in contact with the end face of the core, andpropagates back down the optical fiber by total internal reflectionstowards the input/output end; and c) a detector which receives theradiation exiting the input/output end of the optical fiber.
 7. A methodfor evaluating a sample comprising:a) contacting the sample with anin-line transmission-based optical fiber sensor wherein the sensorcomprises an optical fiber having a core and a cladding orcladding/buffer layer surrounding the core, and having an input end andan output end, wherein the optical fiber has a sensing area locatedbetween the input end and output end, and wherein the sensing area isdefined by a layer of surface plasmon resonance supporting metal incontact with at least a portion of the optical fiber core free from thesurrounding cladding or cladding/buffer layer, and wherein the sensingarea of the sensor is in contact with the sample; b) directing a sourceof electromagnetic radiation of multiple wavelengths into the input endof the optical fiber core such that the radiation propagates from theinput end towards the output end of the optical fiber core by totalinternal reflections, and wherein the propagating radiation interactswith the sensing area of the sensor which is in contact with the sample;and c) measuring the radiation exiting the output end of the opticalfiber.
 8. A method for evaluating a sample, comprising:a) contacting thesample with a terminated reflection-based optical fiber surface plasmonresonance sensor wherein the sensor comprises an optical fiber having acore and a cladding or cladding/buffer layer surrounding the core, andhaving an input/output end and a terminal reflection end, wherein theterminal reflection end is defined by an end face of the core in contactwith a reflective layer that does not support surface plasmon resonance,wherein the optical fiber has a sensing area located between theinput/output end and terminal reflection end or at the terminalreflection end, and wherein the sensing area is defined by a layer ofsurface plasmon resonance supporting metal in contact with at least aportion of the optical fiber core free from the surrounding cladding orcladding/buffer layer, and wherein the sensing area of the sensor is incontact with the sample; b) directing a source of electromagneticradiation of multiple wavelengths into the input/output end of theoptical fiber core such that the radiation propagates from theinput/output end towards the terminal reflection end by total internalreflections, internally reflects off the reflective layer in contactwith the end face of the core, and propagates back down the opticalfiber core by total internal reflections towards the input/output end,and wherein the propagating radiation interacts with the sensing area ofthe sensor which is in contact with the sample; and c) measuring theradiation exiting the input/output end of the optical fiber.
 9. A fiberoptic surface plasmon resonance system, comprising:a) an in-linetransmission-based fiber optic surface plasmon resonance sensor whereinthe sensor comprises an optical fiber having a core and a cladding orcladding/buffer layer surrounding the core, and having an input end andan output end, wherein the optical fiber has a sensing area locatedbetween the input end and output end, and wherein the sensing area isdefined by a layer of surface plasmon resonance supporting metal incontact with at least a portion of the optical fiber core free from thesurrounding cladding or cladding/buffer layer; b) a source ofelectromagnetic radiation of multiple wavelengths whose output isapplied to the input end of the optical fiber such that the radiationpropagates from the input end towards the output end by total internalreflections; and c) a spectral intensity distribution detector whichreceives the radiation exiting the output end of the optical fiber. 10.The fiber optic surface plasmon resonance system of claim 9 wherein thedetector comprises a spectrograph.
 11. The fiber optic surface plasmonresonance system of claim 10 wherein the spectrograph comprises acomputer.
 12. A fiber optic surface plasmon resonance system,comprising:a) an in-line transmission-based fiber optic surface plasmonresonance sensor wherein the sensor comprises an optical fiber having acore and a cladding or cladding/buffer layer surrounding the core, andhaving an input end and an output end, wherein the optical fiber has asensing area located between the input end and output end, and whereinthe sensing area is defined by a layer of surface plasmon resonancesupporting metal in contact with at least a portion of the optical fibercore free from the surrounding cladding or cladding/buffer layer; b) asource of electromagnetic radiation of multiple wavelengths whose outputis applied to the input end of the optical fiber such that the radiationpropagates from the input end towards the output end by total internalreflections; and c) an analyzer which receives the radiation exiting theoutput end of the optical fiber.
 13. The fiber optic surface plasmonresonance system of claim 12 wherein the analyzer comprises a computer.14. A fiber optic surface plasmon resonance system, comprising:a) aterminated reflection-based fiber optic surface plasmon resonance sensorwherein the sensor comprises an optical fiber having a core and acladding or cladding/buffer layer surrounding the core, and having aninput/output end and a terminal reflection end, wherein the terminalreflection end is defined by an end face of the core in contact with areflective layer that does not support surface plasmon resonance,wherein the optical fiber has a sensing area located between theinput/output end and the terminal reflection end or at the terminalreflection end, and wherein the sensing area is defined by a layer ofsurface plasmon resonance supporting metal in contact with at least aportion of the optical fiber core free from the surrounding cladding orcladding/buffer layer; b) a source of electromagnetic radiation ofmultiple wavelengths whose output is applied to the input/output end ofthe optical fiber, and wherein the radiation propagates from theinput/output end towards the terminal reflection end by total internalreflections, internally reflects off the reflective layer in contactwith the end face of the core, and propagates back down the opticalfiber by total internal reflections towards the input/output end; and c)a spectral intensity distribution detector which receives the radiationexiting the input/output end of the optical fiber.
 15. The fiber opticsurface plasmon resonance system of claim 14 wherein the detectorcomprises a spectrograph.
 16. The fiber optic surface plasmon resonancesystem of claim 15 wherein the spectrograph comprises a computer.
 17. Afiber optic surface plasmon resonance system, comprising:a) a terminatedreflection-based fiber optic surface plasmon resonance sensor whereinthe sensor comprises an optical fiber having a core and a cladding orcladding/buffer layer surrounding the core, and having an input/outputend and a terminal reflection end, wherein the terminal reflection endis defined by an end face of the core in contact with a reflective layerthat does not support surface plasmon resonance, wherein the opticalfiber has a sensing area located between the input/output end and theterminal reflection end or at the terminal reflection end, and whereinthe sensing area is defined by a layer of surface plasmon resonancesupporting metal in contact with at least a portion of the optical fibercore free from the surrounding cladding or cladding/buffer layer; b) asource of electromagnetic radiation of multiple wavelengths whose outputis applied to the input/output end of the optical fiber, and wherein theradiation propagates from the input/output end towards the terminalreflection end by total internal reflections, internally reflects offthe reflective layer in contact with the end face of the core, andpropagates back down the optical fiber by total internal reflectionstowards the input/output end; and c) an analyzer which receives theradiation exiting the input/output end of the optical fiber.
 18. Thefiber optic surface plasmon resonance system of claim 17 wherein theanalyzer comprises a computer.